The present invention generally relates to frequency-modulated electric element control, and more particularly to an apparatus, a method, and computer-readable medium for varying DC power supplied to a heating element.
Heating elements are installed, for example, in home appliances such as ovens, washers, and dryers. In an oven, for example, AC power may be supplied to a bake heating element, a convector heating element, and a broil heating element in order to heat up the air in the oven cavity to a target temperature set by the user of the oven.
FIG. 1 shows a block diagram of components of an exemplary system 10 in the related art for providing AC power to a heating element. The exemplary system 10 includes a user input device 20; a comparator 30; a temperature sensor 40; an AC power supply 50; a switch 60; and a heating element 70.
A user of an oven, for example, may utilize the user input device 20 to set a target temperature Ttarget for the air inside the oven cavity. The user input device 20 may be, for example, a knob or a keypad that is located, e.g., at a front panel of the oven. The target temperature Ttarget may be, for example, in the range from 200° F. to 500° F. The target temperature Ttarget is then provided to the comparator 30, e.g., a microcontroller, which compares the target temperature Ttarget to an actual temperature Tactual of the air inside the oven cavity. The actual temperature Tactual is supplied to the comparator 30 by the temperature sensor 40, which may be located inside or in close proximity to the oven cavity, for example.
FIG. 2a shows an exemplary temperature curve of the actual temperature Tactual in, for example, an oven cavity of the related art. The user may set the target temperature Ttarget at time t0. If, at time t0, the comparator 30 determines that the target temperature Ttarget is higher than the actual temperature Tactual, and if the difference between the target temperature Ttarget and the actual temperature Tactual is equal to or greater than a predetermined amount ΔT, the comparator 30 instructs the switch 60 to switch on the AC power from the AC power supply 50 so that AC power is now supplied to the heating element 70. The switch 60 may be, for example, a proportional-integral-derivative (PID) controller.
The AC power supplied to the heating element 70 may be in the order of 2000 Watts, as shown in FIG. 2b. Since AC power is now supplied to the heating element 70, the heating element 70 heats up and, as a result, the actual temperature Tactual of the air inside the oven cavity rises, as shown in FIG. 2a. This operational mode of the oven may be referred to as the preheating mode.
The temperature sensor 40 periodically senses the actual temperature Tactual and forwards it to the comparator 30 for comparison to the target temperature Ttarget set by the user at time t0. If the comparator 30 determines at time t1 that the target temperature Ttarget is equal to the actual temperature Tactual, as shown in FIG. 2a, the comparator 30 instructs the switch 60 to switch off the AC power to the heating element 70, as shown in FIG. 2b. The oven may now enter an operational mode that may be referred to as a baking mode or cooking mode.
Even though the AC power to the heating element 70 is now turned off, the actual temperature Tactual of the air inside the oven cavity continues to rise for a certain period of time, as shown in FIG. 2a, due to residual heat dissipation from the heating element 70 into the oven cavity.
As the heating element 70 cools down, the temperature sensor 40 continues to periodically sense the actual temperature Tactual and continues to periodically supply the actual temperature Tactual to the comparator 30 for comparison with the target temperature Ttarget. If, at time t2, the comparator 30 determines that the target temperature Ttarget is higher than the actual temperature Tactual and that the difference between the two temperatures is equal to or greater than the predetermined amount ΔT, as shown in FIG. 2a, the comparator 30 once again instructs the switch 60 to switch on the AC power to the heating element 70, as shown in FIG. 2b. This switching on and off of AC power to the heating element 70 now continues until the user turns off the oven. For example, as shown in FIG. 2b, the AC power to the heating element 70 is turned on at times t4 and t6, and turned off at times t3, t5, and t7 in response to the actual temperature curve of FIG. 2a.
As can be seen in FIG. 2b, when the switch 60 turns on the AC power to the heating element 70 at t0, t2, t4, etc., it is always the full AC power of, e.g., 2000 Watts that is applied to the heating element 70. This application of the full AC power leads to high power consumption, in particular during the preheating mode, and to inrush currents to the heating element 70, which is the leading cause for heating element breakdown and, ultimately, heating element failure.
Furthermore, as apparent from FIG. 2a, the switching on and off of the full AC power in the related art leads to overshoots and undershoots of the target temperature Ttarget by relatively large degrees so that the target temperature Ttarget can only be approximated within a certain, relatively large interval. This is because the system 10 waits until the temperature sensor 40 detects a significant difference ΔT between the target temperature Ttarget and the actual temperature Tactual before the switch 60 applies the full AC power to the heating element 70. As noted above, by the time the temperature sensor 40 senses that the actual temperature Tactual equals the target temperature Ttarget, the heating element 70 is fully heated and, even though the switch 60 switches off the AC power to the heating element 70, residual heat in the heating element 70 continues to produce heat in the oven cavity until the heating element 70 cools off. The resulting overshoots and undershoots of the target temperature lead to uneven cooking or baking of the food in the, e.g., oven cavity.